Implant and a method of making the implant and a method of calculating porosity of a porous material

ABSTRACT

A method of making an implant having a porous portion is disclosed. The method comprises the following steps: obtaining an artificial foam containing porous portion; scanning the artificial foam to obtain a digital porous model; editing the digital porous model; assembling the digital porous model to form a digital porous block; editing the digital porous block to obtain a digital implant model; forming the implant by printing the digital implant model through a 3D printer. An implant and a method of calculating porosity a porosity of a porous material are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 16/653,921 filed Oct. 15, 2019. The entire contents of which areincorporated herein by reference.

FIELD OF THE INVENTION

This invention is related to a method of making an implant withreticulated titanium porous layer as well as an implant with reticulatedtitanium porous layer and substrate portion by three-dimensional (3D)printing process.

BACKGROUND OF THE INVENTION

Cementless fixation has been widely used for orthopedic implant for longterm survival rate. Ideal integration of bone into and onto porous layerof implants provides interlocking at implant/bone interface by boneingrowth. Reticulated tantalum foam has similar pore structure as humantrabecular bone and has clinically demonstrated good fixation and boneingrowth results. A pore size of greater than 150 μm facilitates theingrowth of mineralized bone. A pore diameter of 200 μm corresponds tothe average diameter of an osteon in human bone, while a pore diameterof 500 μm corresponds to remodeled cancellous bone. However, tantalum isvery expensive, thus limited its applications.

In contrast, titanium is more cost effective and has been shown to havehigh biocompatibility. A reticulated titanium foam is desirable. Themethods of making porous titanium have been progressed and classifiedinto three categories. The first class was sintered beads and metalmeshes onto solid metallic substrates invented in 1980s. The secondmethod was sintering or welding pre-made metallic foam onto metallicsubstrate invented in 1990s. The third method was powder coating onscaffold and sintering in 2000s, and the third was additivemanufacturing or three-dimensional (3D) printing invented in recentdecades. Since 3D printing method has advantage of making one-stepprocess, it becomes the frontier technology today.

The 3D printing method is divided into into three types. The firstmethod was mathematically calculated porous structure, which usuallyhave regular pore and strut patterns and do not simulate the cancellousbone's random structure. The second method was computer softwarealgorithms to automatically create random porous structures, but thismethod does not result in the same structure as cancellous bone. Thethird method was the direct reverse engineering method, in which theporous metal structure was copied from a local cancellous bone bymicro-CT scan, then 3D printed metal porous structure. The directreverse engineering method has the advantage in replication of localcancellous bone structure.

However, the direct reverse engineering method has two disadvantages.One is a large variation of bone source. Human bone has high variationin porous structure. The cortical bone is very dense structure while thecancellous bone is more porous, but bone density and porosity vary in alarge range. For example, age, gender, and genetics all contribute tovariation in bone structure, in addition to pathologies. Elderlypatients may have must less bone density than younger skeletally maturepatients in their middle age. Patients with Osteoporosis have much lowerbone density than those with no osteoporosis. Another disadvantage ofthe direct reverse engineering method is the properties of the bonestructure captured by this method is location specific and bonespecific. For the same person, the bone porosity depends on locationwithin the bony structure. Humeral bones are more porous than femoralbones, and within each bone, the density may vary exponentially alongthe length of the diaphysis. For a same person and some location, thebone porosity is also dependent how far from bone morrow and corticalbone radially that the image is acquired. The center portion of thecancellous bone near marrow has lower density than the cancellous bonenear to the cortical bone. This variation of bone source andsite-specific nature of this approach makes the direct reverseengineering method difficult to utilize and apply generally to differentimplants and can be expensive to characterize all the myriad variablesto create a representative structure for use in large scale productionof implants.

In addition, bone ingrowth and fixation have been shown to relate to thepore size and interconnectivity of a structure, but no convincingevidence exists that there is a specific advantage of reproducing theexact anatomical structure over simplified structures which have similarphysical characteristics that allow bone ingrowth and vascularization.The extra complexity of the exact anatomic structures adds the need forsignificantly more computational resources and additionally thesestructures are harder to pattern into larger forms and blocks because oftheir anisomery. Because they do not have symmetric properties, thesestructures must scan larger regions which are digitally acquired atlower resolution by necessity or they must scan many multitudes of sitesat high resolution by dissecting and extracting from small blocks of alarger bone which is costly, time consuming, and does not solve thepatterning problem in itself.

There is an additional technical challenge to quantitatively measurestrut thickness, pore diameter, and porosity in real implant component.The current method to measure porous titanium is ASTM F1854-15,“Standard Test Method for Stereological Evaluation of Porous Coatings onMedical Implants”. This method is a 2D method, using two-dimensionalsignal to estimate three-dimensional signal. The method requiresphysically cutting the component and manually measuring strutcross-sections area. This is labor intensive and is prone to errors. Onthe other hand, ASTM F3259-17, “Standard guide for Micro-computedtomography of tissue engineered scaffold.” has been widely used forpolymer and ceramics but is not recommended for use with porous metalsbecause metallic artifacts significantly increase measurement errors.

It is desirable to find a good method to quantitively measure thereticulated titanium foam structure by Micro-CT method.

SUMMARY

It is an object of the present invention to provide a relatively lowcost porous implant and a method of making the implant relativelyinexpensive. Another object of the present invention is to provide amethod capable of more accurately measuring the porosity of a porousmaterial.

In order to achieve the above object, according to one aspect, thepresent invention provides a method of making an implant having a porousportion, comprising:

-   -   obtaining an artificial foam containing porous portion;    -   scanning the artificial foam to obtain a digital porous model;    -   editing the digital porous model;    -   assembling the digital porous model to form a digital porous        block;    -   editing the digital porous block to obtain a digital implant        model;    -   forming the implant by printing the digital implant model        through a 3D printer.

In one embodiment, the step of editing the digital porous modelcomprises editing strut thickness and/or pore diameter of the digitalporous model.

In another embodiment, the step of editing the strut thickness and/orpore diameter in the digital porous model comprises scaling-up orshrinking-down the strut thickness and/or the pore diameter.

In another embodiment, the step of assembling the digital porous modelto form the digital porous block comprises patterning the digital porousmodel.

In another embodiment, the step of assembling the digital porous modelto form the digital porous block comprises patterning the digital porousmodel along three dimension of a Cartesian coordinate, a columncoordinate, or a spherical coordinate.

In another embodiment, the step of assembling the digital porous modelcomprises extracting an elementary porous unit from the digital porousmodel and combining a plurality of elementary porous units to form thedigital porous block.

In another embodiment, the step of editing the digital porous blockcomprises cutting the digital porous block into a digital porous layerand overlaying the digital porous layer onto a substrate to form thedigital implant model.

In another embodiment, the shape of the digital porous layer conforms tothe shape of the implant to be formed, and the substrate conforms to theshape of the implant to be formed.

In another embodiment, the step of overlaying the digital porous layeronto the substrate is accomplished by Boolean intersection.

In another embodiment, the substrate is a solid substrate or a poroussubstrate.

In another embodiment, the artificial foam containing porous portion iscut into a cube geometry prior to scanning.

In another embodiment, the cube has a volume of less than 0.5 cubicinches.

In another embodiment, scanning the artificial foam to obtain a digitalporous model is accomplished by micro-CT.

In another embodiment, the implant is further cleaned after 3D printing.

In another embodiment, the implant is further grit blasted and/or coatedafter 3D printing.

In another embodiment, the reticulated foam is selected from any of thefollowing foams: polyurethane foam, carbon foam, ceramic coated carbonfoam, metal coated carbon foam.

In another embodiment, the reticulated foam is selected from any of thefollowing: aluminum coated carbon foam, copper coated carbon foam,nickel coated carbon foam, silicon carbide coated carbon foam, tantalumcoated carbon foam, titanium nitride coated carbon foam, titaniumcarbide coated carbon foam, chromium coated carbon foam.

According to another aspect, the invention provides an implant, whereinthe implant has a substrate and a porous portion overlapping thesubstrate, the implant being made through the following steps:

-   -   obtaining an artificial foam containing porous portion;    -   scanning the artificial foam to obtain a digital porous model;    -   editing the digital porous model;    -   assembling the digital porous model to form a digital porous        block;    -   editing the digital porous block to obtain a digital implant        model;    -   forming the implant by printing the digital implant model        through a 3D printer.

According to yet another aspect, the invention provides a method ofcalculating a porosity of a porous material, the method comprising thesteps of:

-   -   obtaining a first porosity by measuring a porosity of the porous        material by micro-CT scan;    -   obtaining an actual porosity by multiplying the first porosity        by a porosity calibration factor.

In one embodiment, the porosity calibration factor is obtained by thefollowing steps:

-   -   3D printing a first sample of a porous material and obtaining a        true porosity of the first sample by a gravimetric and        volumetric methodology;    -   3D printing a second sample of the porous material and measuring        the porosity of the second sample by micro-CT scan;    -   the true porosity of the first sample being divided by the        porosity of the second sample so as to obtain the porosity        calibration factor.

In one embodiment, the first sample is a cube, and the second sample isa disc.

As compared to prior art, the novelty of this invention includes thefollowing aspects.

-   -   (1) Selection of reticulated artificial foam for micro-CT scan        instead of human bone in pre-arts. This step avoided using human        cadavers and is not site specific. Foam is more homogenous but        more consistent and represents clinically demonstrated        structures.    -   (2) Scale-up or shrink-down pore structure. A process is applied        to adjust pore structure by scaling-up or shrinking-down the        resulting 3-dimensional file. Pore diameter and strut size can        be tailored to different sites of implants by using one micro-CT        scan. This is contrasting to and more efficient than prior art        (U.S. Pat. No. 8,843,229) that scanned multiple positions of        human cadavers to fit different sites of implants.    -   (3) Digital assembly of foam. A small elementary porous cube        will be used to pattern in X, Y and Z directions to create a        larger structure from which to merge into implant structures. An        overlap may be used to ensure no large asperities or gaps will        be present or the geometry may be mirrored or patterned and then        the larger block may be used to Boolean intersect with the 3D        bodies to create the final form. One advantage of this approach        is the minimization of the size of digital file and cost        effectiveness in producing a structure which has been shown to        meet the criteria for integration and fixation.    -   (4) Creation of the titanium foam structure with 3D printing        technology. For example, Direct Metal Laser Sintering (DMLS) is        used, which is a form of metal 3D printing technology.    -   (5) Micro-CT inspection. A calibration method was discovered to        overcome metal artifacts of micro-CT to more precisely measure        strut thickness, pore diameter, and porosity in three        dimensions.

The above novelties were discovered during the execution of thisinvention. Three unexpected results were found. First, the directreverse engineering approach did not work well for highly porousreticulated foam. The extremely high surface area of the foam and largevolume geometry of orthopedic implants made the digital file too largeto store and operate on using typical workstations and laptop computers.To solve the problem, the inventor first scan a large size ofreticulated foam by high resolution micro-CT scan, then extracted asmall volume as elementary volume as a building block. Using softwaretools, many elementary volumes were patterned into a large block whichis large enough to accept the maximum implant size in the family ofimplant sizes. Then, Boolean geometry was used to create the finalgeometry from the large block by means of an Intersection function. Analternative method may be used by was directly patterning even smallerunit cells extracted from the original scan within a volume.

Another unexpected result was that the strut size, pore diameter, andporosity were not able to be replicated by direct reverse engineering.The commercially available reticulated foams were made for otherengineering applications, not for orthopedics. The pore structures werelimited, the specific human sites of implants were multiple, and theresolution of 3D printing process was limited by laser beam width andother DMLS parameters. These three challenges required a trial-and-errorapproach to obtain a preferred porous structure with the mechanical andstructural parameters that match the needs of the application. Thismethod is cumbersome and needs a lot of experiments. Instead, theinventor used a scale-up or shrink-down approach in digital editingstage for optimization of the pore structure and porosity. As a result,only three experiments obtained the optimal strut size, pore diameters,and porosity.

The third unexpected result was the calibration of micro-CT method.Engineers have struggled for decades to accurately measure porous metalstructure using micro-CT method, due to metal artifacts. The metalartifacts artificially overestimate strut thickness, thus these methodstypically historically resulted in lower pore diameter and porosity thanthe actual porosity. In this invention, a porous titanium cube's trueporosity was accurately measured by using a gravimetric and volumetricmethodology. The porosity ratio is defined as the true porosity dividedby the measured porosity. Once determined empirically by use of a knownporous structure, this calibration factor can be applied to the titaniumspecimens to accurately calibrate strut thickness, pore diameter, andporosity of the structure. The calibrated data were consistent with thedirect experimentally measured results.

The novelty and Inventiveness of this invention will be furtherdescribed in the detailed descriptions and examples.

BRIEF DESCRIPTION OF THE INVENTION

FIG. 1 is a diagram illustrating a reticulated titanium porousstructure.

FIG. 2 is a flow chart illustrating a process of making an implant.

FIG. 3 is a diagram illustrating Micro-CT scanning of a reticulatedartificial foam and pore scaling-up in digital file

FIG. 4 is a diagram illustrating digital assembling a digital cube intoa porous layer.

FIG. 5 is a diagram illustrating porous layer being assembled with solidsubstrate into a digital acetabular shell.

FIG. 6 is a diagram illustrating volume percentage of three-dimensionalpore diameters of 80 PPI reticulated SiC foam by micro-CT method.

FIG. 7 is a diagram illustrating volume percentage of three-dimensionalpore diameters of 80 PPI reticulated SiC foam by micro-CT with 20 Voxelresolution.

FIG. 8 is a diagram illustrating accumulated volume percentage of threedimensional pore diameters of 80 PPI reticulated SiC foam by micro-CTwith 20 Voxel resolution.

FIG. 9 is a diagram illustrating light microscopy of 3D printedreticulated Ti6Al4V foam by with Ti:SiC:=1:1 scale.

FIG. 10 is a diagram illustrating light microscope pictures of 3Dprinted reticulated Ti foam by light microscopy with Ti:SiC=1.25:1scale.

FIG. 11 is a diagram illustrating light microscope pictures of 3Dprinted reticulated Ti foam by light microscopy with Ti:SiC=1.5:1 scale.

FIG. 12 is a diagram illustrating Micro-CT scan of 3D printedreticulated Ti foam, Ti:SiC=1.5:1.0 scale.

FIG. 13 is a diagram illustrating strut thickness distribution ofreticulated Ti6Al4V porous layer with Ti:SiC=1.5:1 scale.

FIG. 14 is a diagram illustrating pore diameter distribution ofreticulated Ti6Al4V porous layer with Ti:SiC=1.5:1 scale.

FIG. 15 is a diagram illustrating pore diameter distribution ofreticulated Ti6Al4V porous layer with Ti:SiC=1.5:1 scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The preferred embodiments of the present invention will be described indetail below with reference to the accompanying drawings, so that thepurposes, features and advantages of the present invention can be moreclearly understood. It should be understood that the embodiments shownin the accompanying drawings are not intended to limit the scope of thepresent invention, and is only used for illustrating the essentialspirit of the technical solution of the present invention.

In the following description, numerous specific details are set forth.However, one skilled in the art may implement embodiments without thesespecific details. In other instances, well-known devices, structures,and techniques that are associated with the present application may notbe shown or described in detail to avoid obscuring the embodiments.

Unless the context indicates otherwise, the words “comprise” andvariations thereof, such as “include” and “have”, are meant to beconstrued as an open, inclusive meaning, that is, not limited to.

FIG. 2 is a flow chart illustrating a process of making an implant. Thefirst step of this invention is the selection of source of porousstructure. In general, all reticulated artificial foams have connectedporous and open pore structure can be selected for micro-CT scan, suchas polyurethane foam, carbon foam, aluminum coated carbon foam, coppercoated carbon foam, nickel coated carbon foam, silicon carbide coatedcarbon foam, tantalum coated carbon foam, titanium nitride coated carbonfoam, titanium carbide coated carbon foam chromium coated carbon foametc. All these foams are commercially available. Tantalum coated carbonfoam has been approved for bone growth, but the atomic weight is so highthat micro-CT will generate a lot of artifacts. Low atomic foams such aspolyurethane foam has low atomic weight, but flexible so the porousstructure easily distorted during holding for Micro-CT scan. Thepreferred reticulated porous foam are ceramics or metal coated carbonfoams with low atomic numbers. The preferred ceramics are such asreticulated Al2O3, mullite, SiC, MgO, CaO, hydroxyapatite, Zirconia etc.The preferred metal foams are aluminum coated carbon foams, coppercoated carbon foam, nickel coated carbon foam, silicon carbide coatedcarbon foam. The most preferred is reticulated SiC coated carbon foam.

Reticulated carbon and SiC coated carbon foams have differentporosities. They are measure by pores per inch (PPI). The commercial SiCfoams have porosity range of 3 PPI, 10 PPI, 20 PPI, 30 PPI, 45 PPI, 65PPI, 80 PPI, and 100 PPI (Ultramet INC, 1217 Montague Street, Pacolma,CA 91331, USA). The lower the number PPI, the larger the diameters ofpores and pores. The carbon foam are fragile, so silicon carbide coatedcarbon forms (briefly called SiC foam) are stronger, especially in highPPI number of foams. In theory, all porous foams are theoreticallyselected as sample of scan. 65 PPI, 80 PPI, and 100 PPI foams arepreferred because they are close to the porosity of cancellous bone.

See FIG. 3 , the second step is micro-CT scanning of reticulated foamand save the scan into a digital foam, that is, save the scan as adigital porous model. Any Micro-CT scanner with high resolution can beused for scanning. The finer the micro-CT scanning, the more detailedstrut structure can be observed. However, the scanning time will belonger, the STL file will be larger. Considering the powder size ofadditive manufacturing are 25 to 100 microns in average diameter, thepreferred micro-CT scan voxel are selected in 20 micrometers to 40micrometers range. The most preferred micro-CT scan voxel is 20micrometers.

The most popular way was direct reverse engineering approach, whichdirectly replicates the porous structure. Table 1 shows the micro-CTscan results of three 80 PPI carbon, 80 PPI SiC, and 65 PPI SiC. FIG.3-5 show the strut thickness and pore distributions of 80 PPI SiC foam.All these three foams have very high open porosity and capable formicro-CT scan to achieve digital files. The average carbon strut was56.4 micron, too fragile to hand. Because SiC was coated on the carbonfoam, the strut thickness increased to about 100 μm, so SiC foam ispreferred. The 80 PPI SiC has 78.7% porosity and all pores are open. Thestrut thickness 98.2±27.2 μm and pores diameter 303±98 μm. Pores over150 μm is more than 93% in volume, which facilitate the ingrowth ofmineralized bone. However, these strut diameter, pore diameter andporosity are limited to the original source of choice.

TABLE 1 STL size, strut thickness, pore diameter, porosity, and openporosity of ½″ reticulated artificial cube by Micro-CT scan. Reticulatedfoams 80 PPI Carbon 80 PPI SiC 80 PPI SiC 65 PPI SiC Voxel of micro-CT20 μm 20 μm 40 μm 40 μm STL file size N/A 440,831 KB 195,500 KB 180,602kB Strut thickness 56.4 ± 17 μm 98.2 ± 27.2 μm 126 ± 40 μm 149 ± 31 μmPore diameter 483 ± 120 μm 303 ± 98 μm 398 ± 94 μm 712 ± 153 μm Openporosity 93.9% 78.7% 76.76% 82.76% Total porosity 93.9% 78.7% 76.76%82.76%

The inventor also tried direct reverse engineering approach for finalgeometry. the inventor customized 80 mm×80 mm×80 mm reticulated Carbonor SiC coated carbon cube, machined the cube into the 68 mm OD and 1.5mm shell, then micro-CT scanned the shell. The cost is very high.

To solve the cost issue, the inventors tried a micro-CT scan 1.0″ SiCfoam with cube geometry (25.4 mm×25.4 mm×25.4 mm). After micro-CTscanning using 20 μm Voxel resolution, the scan engineer tried toextract the scan data to STL file format, but the STL file size was toobig to handle by even a supper computer. Finally, the inventor tried totake out 0.5″ cube (12.5 mm×12.5 mm×12.5 mm) data from the 1.0″ cube,the file size was smaller, then a lap-top computer was successfullysaved the STL file.

The third step of the method is editing the digital porous model, seeFIG. 3 . For example, the dimension and the shape of the digital porousmodel may be edited. Particularly, the strut and pore size of the STLfile of the porous foam was edited. For example, the strut and pore sizeof the STL file of the porous foam was scaled-up or shrink-down. Thefinal printed foam porosity and strut size may be different from any oforiginal scanned foam. This process was scanning one reticulated foamonly, obtaining STL file, then digitally scale-up or shrink-down the STLfile. Based on digital imaging and final 3D printing, to find theoptimum pore and strut structure. This approach can adjust porestructure with unlimited range and fit any position bone in the humanbody.

Table 2 lists a theoretical range of pore sizes based on differentscaling up or shrink-down scale. This value is the guideline for initialselection of SiC or Carbon foam, not final size of 3D printed foam. Intheory, any reticulated artificial foam can be selected as micro-CT scansample to get digital porous structure, through shrink-down (scalefactor <1.0) from larger pore foams such as 3 PPI, 10 PPI, 20 PPI, 30PPI, 40 PPI, or scale-up (scale factor >1.0) such as 80 PPI, 100 PPI.The preferred pore size is close to as desired pore size and makeadjustment through scaling, such as 65 PPI, 80 PPI, and 100 PPI foamswere selected for micro-CT scan.

TABLE 2 Theoretical pore diameter with different scale-up scales. Scalefactor 0.1 0.4 0.6 1.00 1.25 1.50  3 PPI foam 846 μm  20 PPI foam 127 μm508 μm 762 μm  45 PPI foam 225 μm 338 μm 564 μm 705 μm 846 μm  65 PPIfoam 156 μm 234 μm 391 μm 489 μm 587 μm  80 PPI foam 190 μm 317 μm 397μm 476 μm 100 PPI foam 152 μm 254 μm 317 μm 381 μm

The fourth step of the method is digital assembly small porous foam intofinal implant geometry, forming a digital implant model, see FIGS. 4-5 .For example, a STL file of 0.5″ cube after pore structure scale-up orshrink-down was selected as a building block. Many 0.5″ cube STL blockswere digitally overlapped each other to form a larger block by a 3Dediting software. This larger block is a digital porous block. Thedigital porous block was cut into a digital shell with multi holes. Thedigital porous layer was laid on a solid substrate or a porous substratewith overlap to form a final acetabular shell for cement-less fixation,see FIG. 5 .

In theory, any shape of porous foam can be cut out from the digitalporous block. Alternatively, a very small digital block can be cut outfrom the ½″ cube from any coordinate, such as spherical coordinate orcolumn coordinate. The small block can be digitally assembled to anyshape of components. The commercial software are available for thedigital assembly. The examples are Geomagic Wrap®, Materilise Magics®,3D Expert®, etc.

The large digital blocks can be cut to any desired geometry for implantsor bone fillers. The inventor found that the overlap of the STL buildingblock is necessary. If just line-to-line assembly, the 3D printedsamples will have visible line or gap. The digital reconstructionincludes porous layer to solid layer as well. A minimum 1.0 μm overlapis needed, preferred over 10 μm overlap, more preferred over 50 μm, themost preferred over 100 μm overlap.

Based on theoretical teaching of 3D printing and pre-arts, the overlapof STL file were not desirable. It would generate a lot of suspendedoverhanging struts and non-closed loops. The computer software wouldconsider the overlap as errors and need to repair. Surprisingly, theinventers has not found any visual mark, no effect on mechanicalproperties and porosity.

The fifth step of the method was 3D printing. Sending the assembledacetabular cup model to a 3D printer. The 3D printing is thestate-of-the art technology by lay-by-layer melting or sintering processunder laser or e-beam as heating source. The 3D metal printers werecommercially available, such as M2 Cursing DL 400W, ProX DMP320, FARSON271 M, BLT-5310. Any one of them can be used for 3D printing thereticulated titanium porous implant. During this process, a softwareconvert STL file into slicing file, then printed into final geometry.After 3D printing, the final acetabular shell was removed metalsupports, grit blasting, mechanical vibration and ultrasonic cleaning toremove loss powder inside of the porous layer. A post grit blasting andcoating process such as coating hydroxyapatite can be conducted on theporous surface.

The six-step was micro-CT inspection of porous structure. ASTM F3259-17,“Standard guide for Micro-computed tomography of tissue engineeredscaffold” has been widely used for polymer and ceramics with highaccuracy to measure strut size, pore size, and porosity. FIG. 7 showsthe three-dimensional pore diameters of 80 PPI reticulated SiC foam bymicro-CT scan. The measured average 316 μm pore size is consistent tothe theoretical calculation 317 μm in Table 1. However, this method hasnot recommended to metal because the metallic artifacts. The metallicartifacts caused a thicker struts (about 25 μm), smaller pore diameters(about 140 μm), and lower porosity (about 15%) than their real value(Table 3). The metal artifacts traditionally minimized by increasingX-ray energy, but still generated a large error, thus not recommended byASTM for measure porous titanium porous layer yet.

TABLE 3 Reticulated Ti6Al4V porous structure measured by micro-CT andlight microscopy before artifacts calibration. Example 1 Example 2Example 3 Strut thickness (micro-CT), μm 260 ± 53 258 ± 56 236 ± 58Strut thickness (Light microscopy), μm 239 ± 38 226 ± 40 213 ± 58 PoreDiameter (Micro-CT, 3D), μm 242 ± 84  325 ± 118  396 ± 155 Pore Diameter(Light microscopy, 2D), μm 350 ± 60 458 ± 68 562 ± 82 Total Porosity(Micro-CT), % 35.5 46.82 56.13 Total Porosity (weight and volume), %50.38 Open porosity/Total porosity (Micro-CT), % 100 100 100

Instead avoiding the artifacts like pre-arts, the inventor used theartifacts as a calibration tool. The inventor define a calibrationfactor for micro-CT as below:

Porosity calibration factor=porosity of first sample (weight andvolume)/porosity of second sample (micro-CT)

Pore diameter calibration factor=Porosity calibration factor

Strut thickness calibration factor=(1/Pore diameter calibrationfactor)^(1/3)

Here, a ½″ cube is used as first sample and as calibration. The trueporosity of the ½″ porous cube was 50.38%, which was accurately measuredweight and volume method. The details were described in Example 7. Theporosity calibration factor is 1.419, equal to the ratio of cubeporosity 50.38% divided second sample porosity 35.5%. In this example,the second sample is a disc)Pore diameter calibration factor is equal toPorosity calibration factor, 1.419. Strut thickness calibration factoris equal to (1/1.419)^(1/3)=0.8896. The calibrated results were listedin Table 4, which were consistent with the light microscopy measurementdata.

It should be understood that other shapes than cubes and discs, such ascuboids, ellipses, etc., may be employed to calculate the porositycalibration factor. As long as the first sample is 3D printed and a trueporosity of the first sample is obtained by a gravimetric and volumetricmethodology, and the second sample is 3D printed and a porosity of thesecond sample is measured by micro-CT scan, then the true porosity ofthe first sample being divided by the porosity of the second sample soas to obtain the porosity calibration factor.

TABLE 4 Reticulated Ti6Al4V porous structure measured by micro-CT andlight microscopy after artifacts calibration. Example 1 Example 2Example 3 Calibrated Strut thickness (micro-CT), μm 231 ± 47 229 ± 50210 ± 52 Strut thickness (Light microscopy), μm 239 ± 38 226 ± 40 213 ±58 Calibrated Pore Diameter (Micro-CT, 3D), μm  343 ± 119  461 ± 167 561 ± 220 Pore Diameter (Light microscopy, 2D), μm 350 ± 60 458 ± 68562 ± 82 Total Porosity (Micro-CT), % 35.5 46.82 56.13 Calibrated Totalporosity (Micro-CT), % 50.38 66.44 79.65 Open porosity/Total porosity(Micro-CT), % 100 100 100

EXAMPLES Example 1. Reticulated Porous Titanium Sample with 1:1 ScaleRatio to SiC Foam

A reticulated titanium porous foam with 1:1 ratio to 80 PPI SiC foam wasmade according to process in FIG. 2 . The 80 PPI SiC foam was made byUltramet INC (1217 Montague Street, Pacolma, CA 91331, USA). The SiCfoam has porosity 80 pores per inch (PPI). A foam block with dimensionof 30 mm×20 mm×12.5 mm was scanned by micro-CT at a voxel of 20 micronsat Microphonics INC (1550 Pond Road, Suite 110, Allentown, PA 18104,USA). The micro-CT scan parameters of SiC foam were Bruker SkyScan 1173micro-CT, voxel 20 μm, source voltage 100 kV, source current 62 μA.Reconstructions were completed using Bruker NRecon software, porosityanalysis was done using Bruker CTAn software and STL models were madeusing Synopsis Simpleware software. There was no ring artifectcorrection, smoothing. Beam hardening correction 40%.

A ½″ cube (12.5 mm×12.5 mm×12.5 mm) was digitally cut out from theoriginal 30 mm×20 mm×12.5 mm block. The micro-CT scan was saved as STLformat. Using Materalise Magics™ 3D printing software, four ½″ porouscubes were digitally assembled into a 1.0″ porous cube (25.4 mm×25.4mm×25.4 mm). The contact of the cubes was face-to-face contact, i.e., nooverlap. Cut the 1.0″ digital cubes into a disc shape with 25.4 mmdiameter and 1.5 mm thickness. The porous disc was digitally assembledwith a solid sample with a thickness of 25.4 mm and 6.25 mm thickness.The porous layer was overlapped 100 μm so as to form a digital model.

The digital model was printed using Ti6Al4V ELI powder by M2 Coursing 3Dprinting machine at GE Additive INC (101 North Campus Drive, FindlayTownship, PA 15126, USA). During printing, the laser beam was set up 150μm diameter. Loose powders were removed from the printed samples powdersby mechanical vibration in air, followed ultrasonic cleaning in water.

The light microscopy showed gaps at the assembly lines. The strutsthickness and pore diameter were analyzed by a digital light microscopyand Micro-CT. The micro-CT scan parameters of Ti6Al4V porous layer (1.5mm thickness, 25.4 mm diameter) on 1.0″ diameter solid were BrukerSkyScan 1173 micro-CT, voxel 20 μm, source voltage 130 kV, sourcecurrent 60 μA.

Reconstructions were completed using Bruker NRecon software, porosityanalysis was done using Bruker CTAn software and STL models were madeusing Synopsis Simpleware software. For minimizing metal artifacts, ringartifacts correction was grade 4, smoothing grade 2, beam hardeningcorrection 100%.

Example 2. Reticulated Porous Titanium Sample with 1.25:1 Scale Ratio toSiC Foam

All process parameters were the same as Example 1, except the digitalSTL file of 80 PPI SiC foam was magnified to 1.25 scale inthree-dimensional geometry.

Example 3. Reticulated Porous Titanium Sample with 1.50:1 Scale Ratio toSiC Foam

All process parameters were the same as Example 1, except the digitalSTL file of 80 PPI SiC foam was magnified to 1.5 scale inthree-dimensional geometry.

Example 4. Reticulated Porous Titanium Cube with 1:1 Scale Ratio to SiCFoam

All process parameters were the same as Example 1, except the ½″ digitalcube was directly printed out into a Ti6Al4V porous cube.

Example 5. Reticulated Porous Titanium Shell with 1:1 Scale Ratio to SiCFoam

All process parameters were the same as Example 1, except digitallyassembly a ½″ porous cube (12.5 mm×12.5 mm×12.5 mm) into a porousacetabular shell with dimension of 40 mm in dimeter, 1.5 mm in thickness(FIG. 4 ). The porous acetabular shell was integrated the porous shellwith a solid substrate into a digital acetabular cup (FIG. 5 ). Thedigital acetabular cup model was sent M2 Cursing 3D printer (GEadditive) to print a physical Ti6Al4V cup. Different from samples(Example 1-3), the face-to-face contact among the cubes were eliminated.Instead, all cubes were digitally assembled with 100 μm overlap, thesame overlap to solid substrate. After 3D printing, there were no visualassembly line in the porous layer.

Example 6. Reticulated Porous Titanium Steroid with 1:1 Scale Ratio toCarbon Foam

All process parameters were the same as Example 1, except an steroidshaped reticulated carbon foam was micro-CT scanned for 3D printing areticulated porous titanium steroid with 1:1 scale ratio. The porousstructure of reticulated carbon foam was shown in Table 1. Due to theextremely fine strut thickness 56.4±17 μm, the 3D printer software washard to recognize the struts. After carful adjust 3D printer parameters,a reticulated porous titanium steroid was printed out, but there were alot of powders left inside the porous space. The printing was notsuccessful.

Example 7. Characterization of SiC Foams and 3D Printed Components

Reticulated porous SiC foam were characterized by micro-CT scanning.Micro-CT was conducted at Microphonics INC (1550 Pond Road, Suite 110,Allentown, PA 18104, USA) according to ASTM F3259-17. The Micro-CTmachine was Skyscan1173. The analysis used adaptive mode (mean of minand max values) with lower grey thresholding 20 and upper greythresholding 150. FIG. 6-8 showed the characteristics of 80 PPI SiC foamstructure, including strut thickness distribution, pore sizedistribution, and accumulated pore size distribution. Table 1 summarizedresults. Average pore size was 300-400 μm and open porosity of 76-78%.Because SiC coated carbon foam is ceramic with low atomic numbers, themetal artifact is negligible. The pore diameter 300-400 μm is consistentto the calculated pore diameter of 317 μm for 80 PPI number in Table 2.

The Reticulated porous titanium porous layer on samples (Example 1-3)and ½″ cube (Example 4) were characterized by micro-CT scanning.Micro-CT was conducted at Microphonics INC (1550 Pond Road, Suite 110,Allentown, PA 18104, USA) according to ASTM F3259-17. The Micro-CTmachine was Skyscan1173. The analysis used adaptive mode (mean of minand max values) with lower grey thresholding 60 and upper greythresholding 155. Because of titanium has metal artifacts for Micro-CT,light microscopy method was used to measure strut thickness and poresize. The weight and volume method were used to direct measure porosityof the ½″ porous Ti6Al4V cube. The weighted was measured by a calibratedanalytical balance with accuracy 0.1 mg. The volume was measured by acalibrated micrometer with accuracy 0.01 mm. Table 3, and Table 4 listreticulated Ti6Al4V porous structure measured by micro-CT and lightmicroscopy before and after artifacts calibration.

FIG. 9-11 show the strut thickness of 3D printed reticulated poroustitanium layers on solid titanium samples by light microscopy picturesof (Example 1-3). The struts thickness is about 220 μm with standarddeviation about 50 μm, even though the titanium porous structures werescaled up to 1.25 and 1.5 scales relative to 80 PPI SiC foam. This meansthat the strut thickness was dominated by the laser beam diameter. Theinitial digital SiC foam struts thickness was 98 μm at 1:1 ratio (Table1), the digital SiC struts were 122 μm and 147 μm after scale-up to 1.25and 1.5 times, which were less than the minimum laser beam size setting150 μm. The Micro-CT measured struts were thicker than the lightmicroscope measured value about 20-30 μm due to metal artifacts (Table1), but the artifacts were corrected based on the teaching of thisinvention.

FIG. 9-11 show the pore diameters of 3D printed reticulated poroustitanium layers on solid titanium samples by light microscopy picturesof (Example 1-3). In contrast to strut thickness, the pore diametersincreased with Ti:SiC scale ratio. The light microscopy measured porediameters are 350±60 μm, 458±68 μm, and 562±82 μm (Table 3), which arecorresponding to designed Ti:SiC scale ratio of 1, 1.25 and 1.5. Thedesigned scale ratios are consistent to the experimental ratio of 1.0(350/350), 1.30 (458/350), and 1.60 (560/350).

FIG. 12 shows the micro-CT scan of 3D printed reticulated Ti foam withTi:SiC=1.5:1.0 scale. The pores were interconnected. FIG. 13-15 showedoriginal micro-CT scan data of strut and pore diameter distributionswithout metal artifacts calibration. The all pores are larger than 180μm and less than 800 μm. Based on U.S. Pat. No. 5,282,861, the bone ispreferred to growth in about 600 μm titanium pores in 400 μm, 600 μm and800 μm range. Therefore, the preferred pore size in this invention wasthe sample Example 3, which has Ti:SiC scale ratio 1.5.

Table 4 listed the porosity of reticulated titanium porous layer. Afterartifacts calibration, the porosities are corrected to 50.38%, 66.44%and 79.65% for Example 1-3. Based on previous discussion, the scale-upprocess was primarily for magnification of pore diameter. Thetheoretical porosity value of Example 1-3 was 50.38%, 62.98%, and75.57%, which very consistent to the calibrated porosities. Theseporosity value indicated that Ti:SiC 1:1 ratio is acceptable, preferred1.25:1 ratio, the most preferred 1.5:1 ratio.

Table 5 shows the bond strength results of reticulated Ti6Al4V ELIsamples. Tensile and shear bond strength were all above the FDA requiredminimum requirement, 20 MPa for tensile and 22 MPa for shear. Allfailure occurred at porous titanium/adhesive interface.

TABLE 5 Bond strength results of reticulated Ti6Al4V ELI samplesReticulated Ti samples Example 1 Example 2 Example 3 Shear Bondstrength, MPa 47.9 ± 3.1 49.3 ± 0.7 42.3 ± 0.8 Failure mode 100%adhesive 100% adhesive 100% adhesive Tensile Bond strength, MPa 76.4 ±4.0 73.4 ± 6.4 70.6 ± 2.5 Failure mode 100% adhesive 100% adhesive 100%adhesive

Preferable embodiments of the invention have been described in detail asabove. It should be understood that, after reading the above teaching ofthe invention, various changes or modifications of the invention can bemade by those skilled in the art. All of the equivalents fall in theprotection scope defined by the attached claims.

What is claimed is:
 1. An implant, wherein the implant has a substrateand a porous portion overlapping the substrate, the implant being madethrough the following steps: obtaining an artificial foam containingporous portion; scanning the artificial foam to obtain a digital porousmodel; editing the digital porous model; assembling the digital porousmodel to form a digital porous block; editing the digital porous blockto obtain a digital implant model; and forming the implant by printingthe digital implant model through a 3D printer.
 2. The implant accordingto claim 1, wherein the step of editing the digital porous modelcomprises editing strut thickness and/or pore diameter of the digitalporous model.
 3. The implant according to claim 1, wherein the step ofediting the strut thickness and/or pore diameter in the digital porousmodel comprises scaling-up or shrinking-down the strut thickness and/orthe pore diameter.
 4. The implant according to claim 1, wherein the stepof assembling the digital porous model to form the digital porous blockcomprises patterning the digital porous model.
 5. The implant accordingto claim 1, wherein the step of assembling the digital porous model toform the digital porous block comprises patterning the digital porousmodel along three dimension of a Cartesian coordinate, a columncoordinate, or a spherical coordinate.
 6. The implant according to claim1, wherein the step of assembling the digital porous model comprisesextracting an elementary porous unit from the digital porous model andcombining a plurality of elementary porous units to form the digitalporous block.
 7. The implant according to claim 1, wherein the step ofediting the digital porous block comprises cutting the digital porousblock into a digital porous layer and overlaying the digital porouslayer onto a substrate to form the digital implant model.
 8. The implantaccording to claim 7, wherein the shape of the digital porous layerconforms to the shape of the implant to be formed, and the substrateconforms to the shape of the implant to be formed.
 9. The implantaccording to claim 7, wherein the step of overlaying the digital porouslayer onto the substrate is accomplished by Boolean intersection. 10.The implant according to claim 7, wherein the substrate is a solidsubstrate or a porous substrate.
 11. The implant according to claim 1,wherein the artificial foam containing porous portion is cut into a cubegeometry prior to scanning.
 12. The implant according to claim 11,wherein the cube has a volume of less than 0.5 cubic inches.
 13. Theimplant according to claim 1, wherein scanning the artificial foam toobtain a digital porous model is accomplished by micro-CT.
 14. Theimplant according to claim 1, wherein the implant is further cleanedafter 3D printing.
 15. The implant according to claim 1, wherein theimplant is further grit blasted and/or coated after 3D printing.
 16. Theimplant according to claim 1, wherein the artificial foam is areticulated foam selected from any of the following foams: polyurethanefoam, carbon foam, ceramic coated carbon foam, metal coated carbon foam.17. The implant according to claim 1, wherein the artificial foam is areticulated foam is selected from any of the following: aluminum coatedcarbon foam, copper coated carbon foam, nickel coated carbon foam,silicon carbide coated carbon foam, tantalum coated carbon foam,titanium nitride coated carbon foam, titanium carbide coated carbonfoam, chromium coated carbon foam.
 18. A method of calculating aporosity of a porous material, the method comprising the steps of:obtaining a first porosity by measuring a porosity of the porousmaterial by micro-CT scan; obtaining an actual porosity by multiplyingthe first porosity by a porosity calibration factor.
 19. The calculationmethod according to claim 18, wherein the porosity calibration factor isobtained by the following steps: 3D printing a first sample of a porousmaterial and obtaining a true porosity of the first sample by agravimetric and volumetric methodology; 3D printing a second sample ofthe porous material and measuring the porosity of the second sample bymicro-CT scan; the true porosity of the first sample being divided bythe porosity of the second sample so as to obtain the porositycalibration factor.